1. Field of the Invention
The present invention relates generally to expandable anchoring tools used in drilling operations. Further, the present invention relates to a method and apparatus for drilling a secondary borehole from an existing borehole in geologic formations. More particularly, this invention relates to a relatively small diameter apparatus which can be run into a borehole through a smaller tubing or otherwise restricted section and then expanded to set within a section of larger diameter casing to perform downhole well operations.
2. Description of Related Art
Once a petroleum well has been drilled and cased, it is often necessary or desired to drill one or more additional wells that branch off, or deviate, from the first well. Such multilateral wells are typically directed toward different parts of the surrounding formation, with the intent of increasing the output of the well. The main well bore can be vertical, angled or horizontal. Multilateral technology can be applied to both new and existing wells.
In order to drill a new borehole that extends outside an existing cased wellbore, the usual practice is to use a work string to run and set an anchored whipstock. The upper end of the whipstock comprises an inclined face. The inclined face guides a window milling bit laterally with respect to the casing axis as the bit is lowered, so that it cuts a window in the casing. The lower end of the whipstock is adapted to engage an anchor in a locking manner that prevents both axial and rotational movement.
Multilateral technology provides operators several benefits and economic advantages. For example, multilateral technology can allow isolated pockets of hydrocarbons, which might otherwise be left in the ground, to be tapped. In addition, multilateral technology allows the improvement of reservoir drainage, increasing the volume of recoverable reserves and enhancing the economics of marginal pay zones. By utilizing multilateral technology, multiple reservoirs can be drained simultaneously. Thin production intervals that might be uneconomical to produce alone become economical when produced together with multilateral technology. Multiple completions from one well bore also facilitate heavy oil drainage.
In addition to production cost savings, development costs also decrease through the use of existing infrastructure such as surface equipment and the well bore. Multilateral technology expands platform capabilities where slots are limited and eliminates spacing problems by allowing more drain holes to be added within a reservoir. In addition, by sidetracking damaged formations or completions, the life of existing wells can be extended. Laterals may be drilled below a problem area once casing has been set, thereby reducing the risk of drilling through troubled zones. Finally, multilateral completions accommodate more wells with fewer footprints, making them ideal for environmentally sensitive or challenging areas.
Often however, a well bore is configured such that a tubular string of a smaller diameter is contained within a larger pipe string or casing, making it necessary to run well tools through the smaller diameter tubular and thereafter perform down hole operations (such as sidetracking) within the larger area provide by the larger tubular or casing. An apparatus and method are herein disclosed which allow a relatively small diameter assembly to be run into a borehole through a smaller diameter tubular or similar restriction and set in a relatively large diameter casing. Generally, such operations are known as thru tubing operation. Disadvantages of thru tubing tools known in the prior art include limited radial expansion capabilities and limited ability to securely anchor within the larger tubular diameter. It has been found that conventional thru tubing whipstock supports may be susceptible to small but not insignificant amounts of movement. Hence, it is desired to provide an anchor and whipstock apparatus that effectively prevent an anchored whipstock from moving. These disadvantages of the prior art are overcome by the present invention.